There is a bivalve mollusk which burrows into submerged timber, such as the hulls of wooden ships, or the beams of piers and jetties. This is called the teredo, or ship-worm, and certainly it does look much more like a worm than a mollusk, for it has a cylinder-shaped body something like a foot in length, with a forked tail, while the shell only covers just a little part at one end. How it burrows into the wood nobody quite knows. It is generally supposed to do so by means of the foot. But in a very short time it will honeycomb a great beam of timber with its burrows, which it always lines with a kind of shelly deposit, weakening it to such a degree that at last it gives way beneath the slightest pressure.
Like a great many other mollusks, the teredo passes through a kind of caterpillar stage before it reaches its perfect form. While it is in this condition it is able to swim freely about in the water, and looks rather like a very tiny hedgehog, being almost globular in shape, and covered all over with short projecting hairs. It is by means of the action of these hairs upon the water that it is able to swim.
CHAPTER XXXVIII
ANNELIDS AND CŒLENTERATES
The important class of the annelids contains those creatures which we generally call worms. There are a great many of these, but we shall only be able to mention one or two.
The Common Earthworm
This worm is really a most interesting as well as a most useful animal. The way in which it crawls is decidedly curious. On the lower part of every one of the rings of which its body is made up, with the sole exception of the head, are four pairs of short, stiff, little bristles, projecting outward from the skin. The worm really hitches itself along by means of these bristles. First it takes hold of the ground with those underneath the front rings, then it draws up its body and takes hold with those underneath the hind ones, and then it pushes its head forward and repeats the process; and so on, over and over again.
If you take a worm and pass it between your finger and thumb from the tail-end toward the head, you can feel these little bristles quite easily.
A worm does not often leave its burrow, however, but generally keeps the tip of its body just inside the entrance, so that it can retreat in a moment in case of danger.